(129e) Radioisotope Technology - Benefits & Limitations in Packed Bed Tower Diagnostics

The use of radioisotope technology in Chemical, Petrochemical and Refinery Process Plant mass transfer vessels has continuously been developed over the past 50 years to assist engineers in troubleshooting process problems, optimize production and plan shutdown schedules to minimize plant down time. Many advances in data acquisition and processing ability now allow critical information to be provided quickly to ensure the most efficient use of plant equipment.

With the advancement of data gathering and handling it is important to note that fundamental physics behind each and every measurement involving radioisotopes has never changed. As with any technology there are many advantages in its use. However there are also limitations that a process engineer must fully understand so that a possible problem is not overlooked or simply discounted based upon results generated.

Mass transfer design can typically be split into two main categories, trayed and packed beds. Packed beds are becoming increasingly popular. In the case of a trayed tower, the open area is typically less than 10% whereas the open area in a packed bed is greater than 50%. Additionally, packing tends to have a lower liquid hold-up than trays. These characteristics combine to result in pressure drop for a theoretical stage being lower in packing than in trayed systems. This reduction in pressure drop can have a significant impact on tower and plant economics.

As with all situations, packed towers bring their own particular challenges. As an example, the same open area and reduced liquid hold-up that benefits pressure drop may result in lower liquid and vapour velocities, and therefore, an increased susceptibility of packed beds to accumulate solids in systems prone to polymerization or precipitation. Another common issue surrounds mechanical integrity. Packed bed towers are more susceptible to damage as a result of pressure surges as compared to trayed vessels. Finally, a critical consideration in the effective operation of a packed tower is the mechanism for fluid distribution. Poor vapour or liquid distribution can result in a significant efficiency reduction. In fact, the necessity of uniform distribution increases with the number of theoretical plates in a bed. This results in tower internal designers limiting bed length and redistributing process materials at specific and regular intervals.

With the increased use of packed beds, the application of radioisotope technology to troubleshoot problems has become popular. This paper, with the aid of case studies, describes the general principles of the radioisotope diagnostic technologies used in both trayed and packed bed towers that allow operating characteristics to be measured. It shows the benefits that can be gained by using the technologies as well as limitations and in some cases the necessary strategy of radioisotope technology integration to successfully determine the source of a particular mass transfer problem.